It is known to fabricate thermal IR detectors on a silicon substrate comprising a thin membrane layer (made of electrically insulating layers) that is formed by etching of part of the substrate. Incident IR radiation increases the temperature of the membrane—which can be measured by either a thermopile, a resistor, or a diode.
For Example, Schneeberger et al “Optimized CMOS Infrared Detector Microsystems,” Proc IEEE Tencon 1995, reports fabrication of CMOS IR detectors based on thermopiles. The thermopile comprises several thermocouples connected in series. KOH is used to etch the membrane and improve the thermal isolation. Each thermocouple comprises 2 strips of different materials, connected electrically and forming a thermal junction at one end (termed hot junction) while the other ends of the material are electrically connected to other thermocouples in series forming a thermal cold junction. The hot junctions of the thermocouples are on the membrane, while the cold junction is outside the membrane. Three different designs of the thermocouples are given in the paper with different material compositions: Aluminium and p-doped polysilicon, Aluminium and n-doped Polysilicon, or p-doped polysilicon and n-doped polysilicon. Incident IR radiation causes a slight increase in temperature of the membrane. The Seebeck effect causes a slight voltage difference across each thermocouple—resulting in a much large increase in voltage difference across the thermopile which is the sum of the voltages across each thermocouple.
Previously, Nieveld “Thermopiles Fabricated using Silicon Planar Technology,” Sensors and Actuators 3 (1982/83) 179-183, showed the fabrication of a thermopile on a micro-chip based on aluminium and single crystal silicon P+ as the materials in the thermocouple. It should be noted that this was a general thermopile device—not intended for IR detection and the thermopile was not on a membrane.
Allison et al., “A bulk micromachined silicon thermopile with high sensitivity,” Sensors and Actuators A 104 2003 32-39, describes a thermopile based on single crystal silicon P-doped and N-doped materials. However, these are formed by wafer bonding of a P-type wafer and an N-type wafer and is also not specifically for use as an IR detector. The fabrication method is also very expensive.
Lahiji et al., “A Batch-fabricated Silicon Thermopile Infrared Detector,” IEEE Transactions on Electron Devices” 1992, describe two thermopile IR detectors, one based on Bismuth-antimony thermocouples, and the other based on polysilicon and gold thermocouples.
U.S. Pat. No. 7,785,002 describes an IR detector with a thermopile based on P and N doped polysilicon. Langgenhager et al. “Thermoelectric Infrared Sensors by CMOS Technology,” IEEE EDL 1992, describes IR detectors comprising thermopiles on a suspended structure comprising aluminium and polysilicon.
Several other Thermopile devices are described by Graf et al. “Review of micromachined thermopiles for infrared detection,” Meas. Sci. Technol. 2007.
Another method of measuring the IR radiation is by the use of thermodiodes. For example, Kim and Chan “A new uncooled thermal infrared detector using silicon diode,” S&A A 89, 2001, describe a diode fabricated by micromachining for use as an IR detector.
Eminoglu et al. “Low-cost uncooled infrared detectors in CMOS process,” S&A A 109 (2003), describes IR detectors made using a CMOS process with diodes on a suspended membrane.
Similarly thermodiode based IR detectors may also be made using an SOI process. However, thermodiodes have the disadvantage that they need a biased voltage or current—which requires power. In addition, it has a high base voltage, which makes it harder to measure small changes in the output voltage.
There have been several reports in literature that suggest that the emissivity/absorptivity of devices can be varied at particular wavelengths by using plasmonic structures, which are periodic structures created on a surface. For example these are described in Shklover et al., “High-Temperature Photonic Structures, Thermal Barrier Coatings, Infrared Sources and Other Applications,” Journal of Computational and Theoretical Nanoscience, Vol 5, 2008, pp. 862-893.
Masuda et al., “Optimization of two-dimensional plasmonic absorbers based on a metamaterial and cylindrical cavity model approach for high-responsivity wavelength-selective uncooled infrared sensors,” Sens. Mater, vol. 26, pp. 215-223, 2014 propose a CMOS thermopile IR detector using a Cr/Au plasmonic absorbing layer for spectral selectivity. However for enhanced thermal isolation of the active area, this latter is physically connected to the substrate only via thin beams. This arrangement not only affects the mechanical robustness of the device, but also poses a limit on the maximum number of thermocouples that can be integrated in the device, thus limiting the overall thermo-electrical conversion efficiency. In Ogawa et al., “Wavelength selective wideband uncooled infrared sensor using a two-dimensional plasmonic absorber,” Optical Engineering, vol. 52, pp. 127104-127104, 2013 also an array of such thermopiles, each having a different plasmonic structure is studied for multicolor IR imaging applications.
A silicon-on-insulator diode uncooled IR focal plane array with through hole plasmonic absorber is reported by Fujisawa et al., “Multi-color imaging with silicon-on-insulator diode uncooled infrared focal plane array using through-hole plasmonic metamaterial absorbers,” in Micro Electro Mechanical Systems (MEMS), 2015 28th IEEE International Conference on, 2015, pp. 905-908. Like for the previous example, through holes are believed to compromise the device mechanical robustness.
In US20150035110A1 by Pisano et al. a MEMS pyroelectric AIN IR detector is reported, where wavelength selectivity is achieved by patterning of the top electrode to form a plasmonic structure. One drawback of pyroelectric detectors in comparison to thermopiles, diodes and bolometers is that they are only sensitive to changes in illumination.
US 20140291704 A1 by Ali et al. describes an Infrared device which can be configured as either an emitter or a source with plasmonic structures placed within the membrane to increase the emission or absorption at a particular wavelength. The patent does not cover the idea of an array of detectors with different patterns of plasmonic structures, nor it covers any differential (or processing) method or transducing technique to analyse the response of several IR detectors having different plasmonic structures and/or compare them to a structure that does not feature a plasmonic pattern.